Water jet boats generally have their water jet impellers connected directly to a motor via a torsion damper combined with a universal joint, or a separate torsion damper and a constant velocity universal joint. Such a system permits the motor to be placed as close as possible to the water jet impeller, thus positioning the motor as far back as possible in the boat. It is preferred to have the motor as close to the stern as possible.
One problem that has always existed with water jet propulsion systems is to keep a boat in a stationary position without shutting off the motor or to clean out the jet intake without shutting off the motor. One solution to this problem is to install a transmission between the impeller and the motor to achieve a neutral position wherein the motor can be warmed up when at anchor or in a marina without creating a water disturbance. This system is also useful when performing tune-ups or checking a high idle, which cannot be done with a direct connection between the impeller and the motor. The transmission system also permits the motor to be used for alternative work such as a front engine power take-off for a generator and the like. Water jets do not need to have a reverse thrust propulsion since the jets have thrust deflectors which are placed at the jet exits to control the direction of water flow or thrust. These deflectors act as a brake for decelerating a boat quickly and also permit reversing a boat when needed.
In most transmissions the input shaft and the output shaft are not in line so the motor has to be mounted in a different position in the boat and this is especially true when a water jet boat is retrofitted with a transmission. Furthermore, a transmission takes up considerably more space and therefore the motor must be placed further towards the bow of the boat which can effect the planing action of a boat. Most water jet impellers are rotated at motor output speed, therefore the transmission has to have a one-to-one ratio which is not a standard transmission and by having a ratio other than one-to-one, the impeller performance is negatively effected. Furthermore, the transmission adds extra weight and requires the presence of oil and oil filters, oil coolers, and raw water heat exchangers to cool oil in the transmission. Also oil temperature and pressure sensors are required together with a mechanical morse cable control to shift the transmission. Furthermore, even a one-to-one transmission has an efficiency loss of from 5% to 10%.
A torsion damper is required between a motor and a water jet impeller to avoid shock and vibration. Motors are generally mounted in a boat on resilient mountings and therefore the torsion damper prevents motor vibration being transferred through to the drive shaft of the impeller and through the impeller bearings to the boat. Universal joints or constant velocity joints take into account movement of a motor on resilient mountings in the boat and provision is made for some slip to occur between the water jet impeller and the motor to allow the engine some freedom of movement in a longitudinal direction.